1. Field of the Invention
The present invention relates to a curtain coating method and a curtain coating apparatus, specifically a curtain coating method and a curtain coating apparatus in which at least one layer of a coating liquid is ejected from a slit, and the ejected coating liquid is made to fall freely from a curtain nozzle lip by using a curtain edge guide, which guides the coating liquid in the form of a curtain liquid film, and bringing the curtain liquid film into contact with a continuously running web so as to form a coating film on the web.
2. Description of the Related Art
Conventionally, a curtain coating method has been proposed as a coating method for use in the production of photosensitive materials such as photographic films. The curtain coating method is a method in which a coating liquid formed of various liquid compositions such as a photographic emulsion (hereinafter, referred to as a coating liquid) is fallen freely to form a curtain liquid film, and then the coating liquid film falling freely is brought into contact with a continuously running web, so as to form a coating film on the web.
Various methods are proposed as the curtain coating method of continuously applying a coating liquid onto a running web. For example, various coating methods, such as blade coating, roll coating, wire bar coating, die coating, etc. have been known.
For example, as shown in FIG. 1A, there is a method which includes ejecting a coating liquid from a nozzle slit of a curtain coating head 90, making the ejected coating liquid 92 fall freely by using a curtain edge guide 91, which guides the coating liquid 92 in the form of a curtain liquid film, so as to form a curtain liquid film, and bringing the curtain liquid film into contact with a continuously running web 94 so as to form a coating film on the web 94, and as shown in FIG. 1B, there is a method which includes ejecting a coating liquid 92 from a slit, moving the ejected coating liquid 92 on a slide surface 97, making the coating liquid 92 fall freely from a curtain nozzle lip 99 by using a curtain edge guide 91, which guides the coating liquid in the form of a curtain liquid film, so as to form a curtain liquid film, and bringing the curtain liquid film into contact with a continuously running web 94 so as to form a coating film on the web 94. The reference sign 93 denotes a suction device, 95 denotes a back-up roll, 96 denotes a slide curtain coating head, 98 denotes a slide edge guide. Also, as for multilayer coating, there is a method which includes ejecting coating liquids with various functions from respective nozzle slits, depositing the ejected coating liquids on a slide surface, so as to form a coating film.
In a slide hopper type curtain coating method as shown in FIG. 1B, it has been known that a so-called teapot phenomenon occurs. The teapot phenomenon is a phenomenon in which a coating liquid flows down on a slide surface and is separated from the curtain nozzle lip 99 located in the lower edge of the slide surface, and upon formation of a curtain film, the curtain film does not fall vertically from a lip edge, but the curtain film was shifted toward the back of the lip. It is considered that the teapot phenomenon prominently occurs when the viscosity of a coating liquid decreases or the amount thereof applied increases, in other words when the Reynolds number is relatively large.
When the teapot phenomenon occurs, a curtain film cannot be guided to the curtain edge guide, and the curtain film cannot be formed. Even when a curtain film is formed by maintaining the curtain film by means of a curtain edge guide, the curtain film does not fall vertically on the surface of the curtain edge guide, and the curtain film is deformed, causing uneven coating.
In order to suppress such teapot phenomenon, for example, in Japanese Patent Application Laid-Open (JP-A) No. 2001-46939 a coating method using a flat glass plate as the curtain edge guide is disclosed. In JP-A No. 2006-55703 there is a description of a curtain coating apparatus in which a flat plate-type edge guide is angled toward the moving direction of the substrate.
JP-A No. 2003-71373 discloses a curtain coating method in which the liquid flow velocity on the slide surface, and the surface tension of the coating liquid are adjusted under predetermined conditions. Japanese Patent Application Publication (JP-B) Nos. 06-51158 and 04-22631 disclose the coating method in which the lip of the curtain nozzle is formed into a predetermined shape.
According to the method described in JP-A No. 2001-46939, by increasing the length of the depth direction of the surface of the curtain edge guide, problems caused by the teapot phenomenon can be solved, specifically, the problem that the curtain film cannot be guided to the curtain edge guide can be solved. However, since the surface of the curtain edge guide is flat, it is difficult to make the curtain film vertically fall down on the guide surface, and deformation easily occurs.
In the device described in JP-A No. 2006-55703, the curtain edge guide is a flat plate similar to that descried in JP-A No. 2001-46939, and deformation on the guide surface is hard to correct. Additionally, due to the angled edge guide, the curtain edge guide needs to be arranged to overhang the base material. Therefore, during coating operation, contaminants are brought to a coating part by means of the base material, and then accumulated between the curtain edge guide and the base material, and such problems occur that the base material is cut, etc.
According to the methods described in JP-A No. 2003-71373 and JP-B Nos. 06-51158 and 04-22631, the occurrence of the teapot phenomenon can be prevented to some extend. However, it can be prevented only under the limited conditions, and these methods cannot respond to the various flow rates and the physical properties of the coating liquid.
Since there is only a narrow and limited space for providing the moving unit in the curtain coating apparatus, it is difficult to provide a complex adjustment mechanism according to various coating conditions. It may be because of this reason, at present, the movement of the curtain edge guide toward the depth direction when the curtain film is seen from the front has not been implemented so far.